What if someone gives you a binary that they claim is built from a particular source code? If you don't decompile it, how do you know if that's true or not? Or what if you can't trust your compiler (a la https://www.win.tue.nl/~aeb/linux/hh/thompson/trust.html)?
Did you build that C compiler yourself? Using what compiler? Unless you bootstrapped it from a handwritten assembler, you'll need to consider the attack outlined in Reflections on Trusting Trust
